The human genome carries an average of 1% to 4% Neanderthal DNA, which means our ancient human ancestors must have interbred with our extinct evolutionary cousins. That raises an obvious next question: why did humans have sex with Neanderthals?

That question isn't meant to be totally glib. The current consensus is that ancient humans must have reproduced with Neanderthals, as otherwise the non-African genome wouldn't feature Neanderthal DNA. But would ancient hunter-gatherer societies, which were hugely reliant on close cooperation between all its members, really have encouraged breeding with a rival group? After all, for humans and Neanderthals to have come into contact, they must have been competing for the same pool of resources. Even if these groups weren't outright fighting each other, would they really have been making babies together?

Researchers C. Michael Barton of Arizona State and Julien Riel-Salvatore of the University of Colorado Denver have put together a new model that traces not just the biological but also the cultural components of the interaction of humans and Neanderthals. While our genomes reveal the biological link between humans and Neanderthals, we don't tend to think of how that process worked in terms of ancient human society, as Barton explains:

"How a culture's working knowledge is passed on is as important as biological information for human evolution. There is a perception that biological evolution determines culture during the Pleistocene era and that cultural influences predominate afterwards (including today). The reality is that the two forces have been working together and they were as important 50,000 years ago as they are today."

Based on their analysis of 120,000 years of archaeological data, Barton and Riel-Salvatore say the most likely reason for the disappearance of the Neanderthals was simply hybridization - humans and Neanderthals started interbreeding, and because there were more humans than Neanderthals, human genes dominated over the generations.

But the most interesting part of their research deals with the cultural side of Neanderthal sex. The researchers considered the very real chance that either humans, Neanderthals, or both discouraged interbreeding. Beyond the fact that the groups were likely competing for resources, there's also the possibility that the two groups saw each other as, well, ugly, making cross-species sex a source of shame or even ostracism.

According to their model, these social taboos could well have existed 50,000 years ago, and it just wouldn't have mattered. Unless the taboos were nearly 100% effective in discouraging interbreeding — and I'm not sure any taboo in human history has ever been anywhere near 100% effective — then humans and Neanderthals would have had more than enough sex to account for our current genetic inheritance. According to Riel-Salvatore, this is the first time such potential taboos have been considered in charting the history of humans and Neanderthals.

The researchers used stone tools to chart how different hunter-gather groups moved throughout Western Eurasia, which was the primary home of Neanderthals. These patterns helped show the intersection of humans and Neanderthals and reveal the timeline of their eventual hybridization. As far as Barton is concerned, the eventual dominance of humans has little to do with genetic superiority over our Neanderthal cousins — and it's very possible this interbreeding happened more than once:

"Other than the fact that they disappeared, there is no evidence that Neanderthals were any less fit as hunter-gatherers of the late Pleistocene than any other human ancestor living at that time. It looks like they were as capable as anyone else. Neanderthals' legacy lives on in our biological genome and possibly in our cultural knowledge. There may have been may other populations like Neanderthals who were integrated into a global human species in the Late Pleistocene. We're the results."